Liquid crystal display devices are widely used for watches, desktop calculators, several kinds of measuring apparatus, panels for automobiles, word processors, electronic note-books, printers, computers, and televisions.
These liquid crystal display devices employ optical anisotropy (.DELTA.n) and dielectric anisotropy (.DELTA..epsilon.) of liquid crystal compounds. As display mode, dynamic scattering (DS) mode, guest-host (GH) mode, twisted nematic (TN) mode, super twisted nematic (STN) mode, thin-film transistor (TFT) mode, and ferroelectric liquid crystal (FLC) mode are known. As their driving mode, static driving mode, time sharing addressing mode, active matrix driving mode, and dual frequency driving mode are known. Among them, TFT mode is being paid public attention in particular.
While several characteristics are required of liquid crystal materials for TFT mode, the following three are considered to be necessary as common characteristics:
1) Temperature range of nematic phase of the liquid crystal materials is wide to such an extent that the materials do not cause the reduction in the temperature range of the nematic phase of a liquid crystal composition when the materials were added to a liquid crystal composition. PA1 2) Viscosity of the liquid crystal materials is low including at a low temperature region. PA1 3) Liquid crystal materials have a large .DELTA..epsilon.. PA1 1. A liquid crystalline compound expressed by general formula (1) ##STR3## wherein R.sub.1 represents an alkyl group having 1 to 10 carbon atoms, ring A represents 1,4-phenylene or 1,4-cyclohexylene, each of X.sub.1, X.sub.2, X.sub.3, and X.sub.4 independently represents hydrogen atom or fluorine atom provided that at least one of X.sub.1 to X.sub.4 is fluorine atom, Y.sub.1 represents CF.sub.3 or OCF.sub.3, and each of m, n, and p is independently an integer of 1 or 0 provided that at least one of m and p is 1. PA1 2. The liquid crystalline compound recited in the aspect 1 mentioned above wherein n is 0, m is 1, X.sub.3 is fluorine atom, and X.sub.4 is hydrogen atom. PA1 3. The liquid crystalline compound recited in the aspect 1 mentioned above wherein n is 1, ring A is 1,4-cyclohexylene, both m and p are 1, and X.sub.3 is fluorine atom. PA1 4. The liquid crystal compound recited in the aspect 1 mentioned above wherein n is 1, ring A is 1,4-phenylene, m is 1, p is 0, all of X.sub.1, X.sub.2 and X.sub.4 are hydrogen atom, and X.sub.3 is fluorine atom. PA1 5. The liquid crystalline compound recited in the aspect 1 mentioned above wherein n is 1, ring A is 1,4-phenylene, m is 0, p is 1, X.sub.1 is fluorine atom, and X.sub.2 is hydrogen atom. PA1 6. The liquid crystal compound recited in the aspect 1 mentioned above wherein n is 1, ring A is 1,4-phenylene, m is 1, p is 0, each of X.sub.1 and X.sub.3 is fluorine atom, and X.sub.2 and X.sub.4 are independently hydrogen atom or fluorine atom. PA1 7. A liquid crystal composition containing at least one liquid crystalline compound defined in any one of the aspects 1 to 6 mentioned above. PA1 8. A liquid crystal composition containing, as a first compound, at least one liquid crystalline compound defined in any one of the aspects 1 to 6 mentioned above, and containing, as a second component, at least one compound selected from the group of the compounds expressed by any one of general formulas (2), (3), and (4) ##STR4## wherein R.sub.2 represents an alkyl group having 1 to 10 carbon atoms, Y.sub.2 represents fluorine atom, chlorine atom, OCF.sub.3, OCF.sub.2 H, CF.sub.3, CF.sub.2 H, or CFH.sub.2, L.sub.1, L.sub.2, L.sub.3, and L.sub.4 represent, independently with one another, hydrogen atom or fluorine atom, Z.sub.1 and Z.sub.2 represent, independently with each other, --(CH.sub.2).sub.2 --, --CH.dbd.CH--, or a covalent bond, and a is 1 or 2. PA1 9. A liquid crystal composition containing, as a first compound, at least one liquid crystalline compound defined in any one of the aspects 1 to 6 mentioned above, and containing, as a second component, at least one compound selected from the group of the compounds expressed by any one of general formulas (5), (6), (7), (8), and (9) ##STR5## wherein R.sub.3 represents fluorine atom, an alkyl group having 1 to 10 carbon atoms, or an alkenyl group having 2 to 10 carbon atoms, any methylene group in the alkyl group or alkenyl group may be replaced by oxygen atom provided that in no case two or more methylene groups are continually replaced by oxygen atom; ring B represents 1,4-cyclohexylene, 1,4-phenylene, or 1,3-dioxane-2,5-diyl, ring C represents 1,4-cyclohexylene, 1,4-phenylene, or pyrimidine-2,5-diyl, ring D represents 1,4-cyclohexylene or 1,4-phenylene, Z.sub.3 represents --(CH.sub.2).sub.2 --, --COO--, or a covalent bond, L.sub.5 and L.sub.6 represent, independently with each other, hydrogen atom or fluorine atom, and b and c are, independently with each other, 0 or 1, ##STR6## wherein R.sub.4 represents an alkyl group having 1 to 10 carbon atoms, L.sub.7 represents hydrogen atom or fluorine atom, and d is 0 or 1, ##STR7## wherein R.sub.5 represents an alkyl group having 1 to 10 carbon atoms, ring E and ring F represent, independently with each other, 1,4-cyclohexylene or 1,4-phenylene, Z.sub.4 and Z.sub.5 represent, independently with each other, --COO-- or a covalent bond, Z.sub.6 represents --COO-- or --C.tbd.C--, L.sub.8 and L.sub.9 represent, independently with each other, hydrogen atom or fluorine atom, Y.sub.3 represents fluorine atom, OCF.sub.3, OCF.sub.2 H, CF.sub.3, CF.sub.2 H, or CFH.sub.2 provided that when Y.sub.3 represents OCF.sub.3, OCF.sub.2 H, CF.sub.3, CF.sub.2 H, or CFH.sub.2, both L.sub.8 and L.sub.9 represent hydrogen atom; and all of e, f, and g are, independently with one another, 0 or 1, ##STR8## wherein R.sub.6 and R.sub.7 represent, independently with each other, an alkyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms, any methylene group in either the alkyl group and alkylene group may be replaced by oxygen atom provided that in no case two or more methylene groups are continually replaced by oxygen atom; ring H represents 1,4-cyclohexylene, 1,4-phenylene, or pyrimidine-2,5-diyl, ring I represents 1,4-cyclohexylene or 1,4-phenylene, Z.sub.6 represents --C.tbd.C--, --COO--, --(CH.sub.2).sub.2 --, --CH.dbd.CH--C.tbd.C--, or a covalent bond, Z.sub.7 represents --COO-- or a covalent bond, ##STR9## wherein R.sub.8 and R represent, independently with each other, an alkyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms, any methylene group in either the alkyl group and alkylene group may be replaced by oxygen atom provided that in no case two or more methylene groups are continually replaced by oxygen atom; ring J represents 1,4-cyclohexylene, 1,4-phenylene, or pyrimidine-2,5-diyl, ring K represents 1,4-cyclohexylene, 1,4-phenylene one or more hydrogen atoms on the ring of which may be replaced by fluorine atom, or pyrimidine-2,5-diyl, ring L represents 1,4-cyclohexylene or 1,4-phenylene, Z.sub.8 and Z.sub.10 represent, independently with each other, --COO--, --(CH.sub.2).sub.2 --, or a covalent bond, Z.sub.9 represents --CH.dbd.CH--, --C.tbd.C--, --COO--, or a covalent bond, and h is 0 or 1. PA1 10. A liquid crystal composition containing, as a first component, at least one liquid crystalline compound defined in any one of the aspects 1 to 6 mentioned above, containing, as a part of a second component, at least one compound selected from the group of the compounds expressed by any one of general formulas (2), (3), and (4) defined in the aspect 8 mentioned above, and containing, as other part of the second component, at least one compound selected from the group of the compounds expressed by any one of general formulas (5), (6), (7), (8), and (9) defined in the aspect 9 mentioned above. PA1 11. A liquid crystal display device composed by using a liquid crystal composition defined in any one of the aspects 7 to 10 mentioned above. PA1 N-I point: 62.8.degree. C. PA1 Viscosity (20.degree. C.): 18.3 mPa.multidot.s PA1 .DELTA.n (25.degree. C.): 0.112 PA1 .DELTA..epsilon. (25.degree. C.): 7.4 PA1 V.sub.th (20.degree. C.): 1.68 V PA1 N-I point: 91.4.degree. C. PA1 Viscosity (20.degree. C.): 25.0 mPa.multidot.s PA1 .DELTA.n (25.degree. C.): 0.198 PA1 .DELTA..epsilon. (25.degree. C.): 7.4 PA1 V.sub.th (20.degree. C.) 1.68 V PA1 N-I point: 88.0.degree. C. PA1 Viscosity (20.degree. C.): 27.0 mPa.multidot.s PA1 .DELTA.n (25.degree. C.): 0.122 PA1 .DELTA..epsilon. (25.degree. C.): 18.1 PA1 V.sub.th (20.degree. C.): 1.24 V PA1 N-I point: 90.9.degree. C. PA1 Viscosity (20.degree. C): 21.9 mPa.multidot.s PA1 .DELTA.n (25.degree. C.): 0.140 PA1 .DELTA..epsilon. (25.degree. C.): 9.2 PA1 V.sub.th (20.degree. C.): 1.79 V PA1 N-I point: 56.8.degree. C. PA1 Viscosity (20.degree. C.): 51.6 mPa.multidot.s PA1 .DELTA.n (25.degree. C.): 0.143 PA1 .DELTA..epsilon. (25.degree. C.): 18.0 PA1 V.sub.th (20.degree. C.): 0.91 V PA1 N-I point: 53.6.degree. C. PA1 Viscosity (20.degree. C.): 29.6 mPa.multidot.s PA1 .DELTA.n (25.degree. C.): 0.109 PA1 .DELTA..epsilon. (25.degree. C.): 9.3 PA1 V.sub.th (20.degree. C.): 1.23 V PA1 DIBAL: Diisobutylaluminum hydride PA1 PTS: p-Toluenesulfonic acid monohydrate PA1 Pd-C: Palladium-carbon PA1 R-Ni: Raney nickel PA1 LAH: Lithium aluminum hydride PA1 FSDF: Fluorosulfonyldifluoro acetic acid methyl ester PA1 DPAE: Ethyl diethylphosphino acetate PA1 Chloranil: Tetrachloro-p-benzoquinone
Among the characteristics mentioned above, 1) includes the requirements that the upper limit of the temperature of nematic phase is high and that the melting point of the liquid crystal materials is low so that the phase separation of the materials due to the precipitation of crystals hardly occurs even at a low temperature region.
Characteristic 2) is an extremely important factor to increase the response speed of oriented liquid crystal molecules to the electric field in a liquid crystal panel (Phys. Lett., 39A, 69 (1972)), and the increase in the response speed is most eagerly demanded at present to improve the quality of the display of liquid crystal compositions. At that time, small dependence on temperature of the response speed mentioned above, that is, small temperature dependence of the viscosity of materials and maintenance of a low viscosity of the materials even at low temperatures are considered to be important from the viewpoint that the deterioration in the quality of the display is not caused even at low temperatures.
Reasons why characteristic 3) is desired are as follows:
In order to actualize the reduction of power consumption and the production of a large image screen, the reduction of driving voltage is necessary. Driving voltage, particularly, threshold voltage (V.sub.th) is a function of .DELTA..epsilon. as expressed by the following equation ##EQU1## wherein k is a proportional constant and K is an elastic constant.
As will be seen from the equation shown above, compounds having a large positive .DELTA..epsilon. is necessary for reducing power consumption.
In addition, a small temperature dependence of V.sub.th is desired even at low temperatures in particular in order to raise the quality of the display of liquid crystal compositions and keep it in a wide temperature range.
In order to respond to such requirements, the exploration for compounds having a large positive .DELTA..epsilon. is being conducted. Among the compounds already discovered, trifluoromethylphenyl derivatives expressed by formula (a) proposed in Laid-open Japanese Patent Publication No. Sho 59-78129 and trifluoromethoxyphenyl derivatives expressed by formula (b) proposed in Laid-open WO Japanese Patent Publication No. Hei 2-501311 are known as examples of the compounds having particularly a large .DELTA..epsilon.. ##STR2##
While these compounds have a three rings structure having CF.sub.3 or OCF.sub.3 at a terminal of the molecule, all of the bonding groups which link the three rings in a straight line are limited to a covalent bond. Based on this fact, in part, those compounds can not completely be said to fully satisfy required characteristics other than .DELTA..epsilon..
Some of the compounds having CF.sub.3 or OCF.sub.3 at a terminal of the molecule are described in Liq. Cryst., 18 (4), 665 (1995), Laid-open WO Japanese Publication Nos. Sho 63-503226, Hei 3-503771, Hei 4-500214, Hei 4-500217, Hei 4-500682, Hei 4-501575, Hei 4-501576, Hei 4-507104, Hei 5-502676, and Hei 6-504032, DE Nos. 4004650 A1, 4013083, 4106345, 4108705 A1, and 4113053 A1, and EP Nos. 0439089 A1 and 0449288, in addition to the publications mentioned above. However, physical parameters of the compounds are not sufficiently described in those publications.
Further, the liquid crystalline compounds used in the liquid crystal compositions for TFT must be stable against external environmental factors such as moisture, air, heat, and light.
In order to develop characteristics required for particular display devices, liquid crystalline compounds are used in the form of a mixture of several or twenty-odd compounds. Accordingly, it is demanded that liquid crystalline compounds have a good miscibility with other liquid crystalline compounds, and a good miscibility even at low temperatures since the use environment of the compounds is expanded recently, in particular.
That is, liquid crystal compositions having a nematic phase particularly at low temperatures, precipitating no crystals, and developing no smectic phase are desired for the liquid crystal compositions for TFT in order to make it possible to use the compositions at a wide range of temperatures. Accordingly, liquid crystalline compounds having an excellent miscibility with other liquid crystalline compounds at low temperatures are particularly desired for the liquid crystalline compounds to be used in TFT.
TFT contains integrated nonlinear devices for switching particular image segments and is one type of active matrix mode liquid crystal displays which are considered to be suitable for high grade information displays such as for televisions, computers, automobiles, and airplanes. Liquid crystal compositions designed for such uses must have an extremely high specific resistance (high voltage holding ratio) and a good UV stability in addition to the large positive .DELTA..epsilon. mentioned above. When liquid crystal compositions having no such characteristics were used, image contrast is decreased with the reduction of electric resistance in liquid crystal panels to raise a problem of "image sticking". Besides, there is a strong tendency of the liquid crystal compositions to shorten their utilization life particularly at the time of driving at a low voltage.
Liquid crystalline compounds similar to those of the present invention are disclosed in U.S. Pat. Nos. 4,871,469 and 5,292,454. However, the compounds disclosed in these patents are distinguished from the compounds of the present invention in their structure. Also, U.S. Pat. Nos. 5,286,411 and 5,516,454 described general formulas of liquid crystalline compounds which encompass some compounds of the present invention. However, the compounds of the present invention are not substantially disclosed in these patents. Besides, the compounds disclosed in these patents have, for example, a low clearing point or a poor capability of exhibiting nematic property compared with those of the present invention.